1. Field of the Invention
The present invention relates to an image formation apparatus and in particular, to an image formation apparatus having an intermediate transfer roller.
2. Description of the Related Art
In general, in an image formation apparatus, an electrostatic latent image formed on a photosensitive body by an electro-photographic process is developed using toner so as to form a toner image. In an image formation apparatus having an intermediate transfer roller, this toner image is temporarily carried by the intermediate transfer roller and then transferred and fixed onto a recording medium to form an image.
That is, in the image formation apparatus in which a visible image is temporarily transferred from the photosensitive body onto the intermediate transfer roller, in order obtain a high quality image formation, it is necessary to preferably perform this transfer from the photosensitive body onto the intermediate transfer roller.
This requires that the intermediate transfer roller maintain a predetermined surface temperature.
There is a conventional technique for providing a built-in heating source in the intermediate transfer roller. However, the intermediate transfer roller is normally covered with resin layer such as a rubber layer and it is difficult to maintain its surface at a predetermined temperature only with the built-in heating source in the intermediate transfer roller. Moreover, it takes a considerable time to increase the surface temperature to the predetermined temperature. Accordingly, such an image formation apparatus shows a slow operation rise, requiring considerable power consumption, and the image quality is not stable.
In order to solve these problems, Japanese Patent Publication A-7-64416 [1] discloses a heating-fixing device for utilizing heat dissipation caused by a resistor-provided fixation film formed as an endless belt and fed along a power supply roller.
This technique enables to maintain at a predetermined value the surface temperature of the pressurizing roller and to reduce the power consumption because only a part of the resistor-provided fixation film is made to dissipate heat.
Moreover, Japanese Patent Publication A-7-49622 [2] discloses an electro-photographic printer comprising transfer-fixation means including two heat rollers, a tension roller, a heated endless belt, and a pressing mechanism. In connection with this, Japanese Patent Publication A-7-49623 [3] discloses an electro-photographic printer comprising transfer-fixation means having auxiliary heating means. Such configurations disclosed in [1] and [2] improve the heating efficiency and maintain the surface temperature of the heating roll at a sufficient temperature for transfer and fixation of a toner image. Note that the disclosures in [1] and [2] relate to a printer using a drum-shaped photosensitive body.
However, there is a problem that the heating-fixing device needs to have a space for feeding the resistor-provided fixation film. This prevents reduction in the apparatus size.
Moreover, in order to obtain a uniform heat application to the surface of the intermediate transfer roller, the heating means requires considerable power consumption.
Furthermore, when a belt-shaped photosensitive body is employed, the surface temperature of the intermediate transfer roller is significantly lowered after operation is started. This is because, heat is radiated from both sides of the belt-shaped photosensitive body and the photosensitive body having a lowered temperature is brought into contact with the transfer assisting roller or the intermediate transfer roller to deprive them of heat. As a result, the surface temperature of the intermediate transfer roller is lowered.